1. Field of the Invention
The present invention relates to an image processing apparatus for processing a tomographic image of the fundus of a subject's eye, and a method for processing the tomographic image.
2. Description of the Related Art
An ocular tomographic imaging apparatus, such as an optical coherence tomograph (OCT), is capable of three-dimensionally observing a state inside the retina layer. In recent years, this tomographic imaging apparatus has attracted attention for the usability for performing disease diagnoses with higher accuracy.
A time domain OCT (TD-OCT) combining a wide-band light source with a Michelson interferometer is a form of the OCT. The TD-OCT scans a delay of a reference arm to measure interference light of backward scattering light of a signal arm and light from the reference arm, thus obtaining information about the depth resolution. However, high-speed image acquisition is difficult to achieve by using such a TD-OCT. Accordingly, as a method for obtaining an image at higher speed, a spectral domain OCT (SD-OCT) is known to obtain an interferogram by using a wide-band light source and a spectroscope. Further, a swept source OCT (SS-OCT) is known to employ a technique for measuring spectrum interference by using a high-speed wavelength sweep light source and a single channel photodetector (as discussed in U.S. Pat. No. 5,321,501). The spectroscope used by the SD-OCT splits and diffracts interference light by using a diffraction grating in space. Therefore, crosstalk interference light may easily occur between adjacent pixels of a line sensor. Interference light from a reflection surface located at a depth position Z=Z0 vibrates at a frequency Z0/n with respect to the number of waves k. Therefore, with increasing depth position Z0 (specifically, with increasing distance from the coherence gate position), an oscillating frequency of the interference light increases, resulting in increased effects of crosstalk of the interference light between adjacent pixels of the line sensor. This means that, with the SD-OCT, performing tomographic imaging at a deeper position causes remarkable degradation in sensitivity. On the other hand, the SS-OCT which does not use a spectroscope is more advantageous in tomographic imaging at a deep position than the SD-OCT.
Further, the spectroscope of the SD-OCT causes a loss of interference light due to the diffraction grating. With the SS-OCT on the other hand, the sensitivity can be easily improved, for example, by performing differential detection of interference light without using a spectroscope. Therefore, the processing speed of the SS-OCT can be improved with an equivalent sensitivity to that of the SD-OCT. The high processing speed enables obtaining a tomographic image having a wide viewing angle.